The Host Rocks Of The Creighton Pluton

Besides the thermal effects of the Creighton pluton, the rocks around the pluton have been affected by a possible thermal event 1950 m.y. ago (Fairbairn and others, 1969; Table I), contact metamorphic effects of the nickel irruptive, and Penokean metamorphism. This complex thermal history makes it difficult to separate the contact metamorphism associated with the Creighton pluton from the thermal effects of other events. The mineral assemblages in the predominantly mafic metavolcanic rocks near the Creighton pluton include quartz, plagioclase, biotite, chlorite, epidote, actinolite, and hornblende. Plagioclase is commonly lacking in amphibole bearing metasedimentary rocks, but occurs with amphibole in mafic metavolcanic rocks. Sericitized pseudomorphs of staurolite are common in metamorphosed pelitic rocks. The mineral assemblages in the metavolcanic rocks in the contact zone of the Creighton pluton are not very different from assemblages in mafic metavolcanic rocks well removed from the pluton. Only where a well developed Se foliation is present can the contact metamorphic effects of the Creighton pluton be sorted out from other events. Although the mineral assemblages observed in both the contact zone of the Creighton pluton and in the regionally-metamorphosed rocks distant from the pluton are broadly indicative of upper greenschist- to lower amphibolite-grade metamorphism, they are not sufficiently distinctive to allow the metamorphic grade to be defined more closely. The present mineral assemblages in the rocks of the Sudbury area are of Penokean age, as shown by the recrystallization and growth of metamorphic minerals in the matrix of the Sudbury breccia.

The dominant structures in the Southern Province rocks of the Sudbury area are Penokean. The structural grain, which trends northeast, is expressed by the strike of foliation, trends of fold axes and the strike of the vertically dipping rocks of the Stobie, Copper Cliff and McKim formations along the southern margin of the Sudbury Basin. Fold axes and foliation in the Whitewater Group, foliation in the nickel irruptive, and the major axis of the Sudbury Basin (Brocoum and Dalziel, 1974) also trend northeast. Penokean foliation is strongest in pelitic rocks such as the McKim Formation and weak or absent in massive rocks such as the Nipissing Diabase, massive metabasalts of the Stobie Formation and portions of the Copper Cliff rhyolite. Fracture cleavages and moderately strong grain-flattening fabrics are common, but strong schistosities are rather rare.

Strong, pre-brecciation foliation occurs in rocks of the Stobie Formation near the contact of the Creighton pluton. Se in the Stobie Formation parallels the contact of the Creighton pluton, and Se within the pluton, and diminishes in intensity with distance from the contact. Se occurs in rocks of all lithologies, but farther from the contact Se commonly is best developed in metasedimentary rocks and weak or absent in massive mafic rocks. Commonly, Se occurs as a strong foliation for some distance from the pluton and then disappears over a short interval. Such a transition, which creates the impression of an abrupt deformation front, is well displayed along the Vermilion River (Fig. 3), where Se is continuously present as a strong, uniform schistosity for 400 meters south of the contact, but is absent from well-exposed rocks 700 meters south of the contact. Linear fabrics (Le) are very rare in the host rocks of the Creighton pluton.

In view of the similar attitudes and age relationships of Se and breccia in both the Creighton pluton and the Stobie Formation, and the restriction of Se to the immediate vicinity of the Creighton granite, it is plausible to regard Se on both sides of the contact as being of the same age. Se locally crosses the contact. In a salient in the granite 4.5 km west of Lively, for example (Fig. 3), the southeasttrending contact is masked by breccia. However, in the nonbrecciated rocks on either side, Se strikes east-west, at a high angle to the contact, This crosscutting relationship, which suggests that Se formed simultaneously on both sides of the contact, is consistent with forceful intrusion of the Creighton pluton. Such local crosscutting relationships, contrasted with the general parallelism between the contact of the Creighton pluton and Se on either side of the contact, indicate that the direction of maximum finite De shortening was controlled by the overall configuration of the pluton and not significantly affected by local irregularities in the contact.

Se generally does not occur beyond a kilometer south of the granite, but along the strike of the Stobie Formation, scattered examples of Se occur up to 4.8 km west and 6.5 km east of the Creighton granite (Fig. 3). Some of the Se examples more remote from the Creighton granite may be effects of local pre-brecciation shearing or faulting. Intrusion effects of the Murray pluton may be responsible for some examples of Se within and near the Murray pluton. The scattered examples of Se at a distance from the Creighton granite are of minor extent and importance compared to the penetrative and ubiquitous foliation within the Creighton granite and its envelope of deformed country rocks.

Se in rocks near the contact is defined by the parallel growth of amphibole (actinolite and hornblende) crystals (Plate 10). Two periods of prebrecciation mineral growth related to the intrusion of the Creighton pluton are evident in metapsammitic rocks from north of Copper Cliff. The earlier period is represented by large hornblende crystals that are internally fractured and display undulose extinction. The early amphiboles have been overgrown and penetrated by smaller, euhedral hornblende crystals which define the strong Se foliation. In this area, the initial formation of hornblende was followed by De deformation and a second, possibly synkinematic, period of mineral growth.

Plate 10. Photomicrograph (crossed nicols) of actinolite greenstone from immediately north of Copper Cliff and about 30 meters from the contact of the Creighton pluton. Strong Se schistosity, defined by the parallel alignment of actinolite crystals, is cut and bent aside by a small breccia veinlet about two millimeters wide. The material in the breccia vein has been altered to epidote, chlorite, and sericite.

Except near the Grenville Front, it is unusual to see more than one set of deformation structures in the Huronian rocks. Crenulation cleavage which locally deforms Penokean foliation may be related to deformation events within the Grenville Province. Crenulation and folding of Se occur in rocks of the Stobie Formation near the contact of the Creighton pluton. Along the Vermilion River (Fig. 3), rocks displaying only a strong Se schistosity 200 meters south of the granite become kinked and finally chevron-folded toward the contact. Chevron folds postdate the breccia and thus are probably of Penokean age. The increasing deformation toward the contact can probably be explained as the result of flattening of the volcanic rocks against the rigid buttress of the granite. Pos-Del folds and crenulation cleavages occur at a number of places along the contact of the Creighton granite. Most are probably Penokean in age, but some may be the result of progressive pre brecciation deformation (De2).

Relationships Between The Nickel Irruptive, The Creighton Pluton, And The Lower Huronian: Constraints On Possible Models Of The Sudbury Basin

Among the factors which must be considered in devising a model for the structure of the Sudbury Basin are the following-(Figs. 1, 2, 3); (1) The annular outcrop pattern of the irruptive (2) The funnel shape of the irruptive in three dimensions (3) The petrographic and density gradations within the irruptive (4) The upward-facing Whitewater Group rocks, which form a doubly-plunging synclinorium within the ring of the irruptive (5) The vertical orientation and southward facing direction of the Stobie and lower Huronian rocks south of the Sudbury Basin. For convenience,. the terms "north range" and "south range", which are commonly used to describe the northern and southern portions of the nickel irruptive, respectively will be used.

The structure of the nickel irruptive at deep levels is unknown. At the
present level of exposure, the form of the irruptive possesses two salient
features which place strong constraints on possible models for the evolution of the Sudbury Basin. First, the outcrop pattern of the irruptive is a complete elliptical ring. Second, the three-dimensional form of the irruptive is that of an asymmetrical funnel: the irruptive along the north range dips about 45� to the south, and the irruptive along the south range dips steeply northward, vertically, or steeply southward (Card, 1968a). A wide variety of intrusive bodies could account for the geometric form of the irruptive: cone-sheet or ring-dike, lopolith, deformed sill, or some more complex intrusive. In addition, some models of the Sudbury Basin (Dietz, 1972) consider the irruptive to have formed as an impact melt and to have been overlain, while still molten, by the Onaping fallback breccia. The geometric constraints which the present structure of the Sudbury Basin imposes on possible evolutionary models apply whether or not the Sudbury Basin is considered to represent a meteor-impact structure.

The composition of the irruptive grades from norite on the outer contact to nearly granitic (granophyre) on the inner contact. This compositional variation is accompanied by a density gradation; the norite has a mean density of 2.86 + 0.09, whereas the mean density of the granophyre is only 2.73 + 0.06 (Popelar, 1972, based on 348 and 211 samples, respectively). Present petrographic models of the Sudbury Basin attribute these compositional and density gradations to post-intrusion gravitational differentiation of the irruptive (Naldrett and others, 1970). Thus it is possible to define a "facing direction" for the irruptive: that is, the norite must have originally underlain the granophyre, though not nec essarily in a strictly horizontal attitude. Everywhere around its circumference, the irruptive presently faces toward the nterior of the Sudbury Basin. Any models which violate these original and final facing conditions must be rejected. The Whitewater Group rocks in the interior of the Sudbury Basin face upward and form a gently folded synclinorium. Although the original relationships between the Whitewater Group and the irruptive are open to question, it is clear that the present facing direction of the Whitewater Group constrains models of the Sudbury Basin in the same way as the facing direction of the irruptive.

The south range of the nickel irruptive and the Whitewater Group rocks immediately north of the irruptive both face north. In contrast, the rocks immediately south of the irruptive face south. This "back to back" relationship (Fig.) is probably the most powerful constraint on models of the evolution of the Sudbury Basin.

Models of the evolution of the Sudbury Basin fall into three broad categories: (1) "deformed sheet" models in which the irruptive is considered to have conconcordantly intruded nondeformed, flat-lying Huronian rocks as a sill-like intrusive, and then deformed into its present form; (2) models in which the irruptive originally cut across flat-lying Huronian rocks at a high angle, with the angular discordance later obliterated through deformation and (3) models in which the irruptive intruded previously deformed Huronian rocks. (Figs. 9, 10, and 11, respectively).

All models of the "deformed sheet" class are inconsistent with the opposite facing directions of the irruptive and the Huronian rocks to the south (Fig. 9). Models of this type which are consistent with one facing direction are inconsistent with the other. If model (2) is correct, the irruptive has moved relative to the rocks to the south, either through rotation of the irruptive and the Huronian rocks into approximate concordance, or through motion along a fault. On the other hand, if model (3) is correct, the irruptive and the rocks to the south have not moved relative to one another since the irruptive was emplaced; two lines of evidence suggest that this model is correct.

Figure 9 (a-g). Geometric constraints on "deformed sheet" models: models of the Sudbury Basin in which the irruptive concordantly intrudes undeformed Huronian rocks. Not drawn to scale. North on left in figures.

Figure 9a. Schematic cross-section of the Sudbury Basin at present, showing the irruptive in a synclinal configuration. Symbols are the same as in Figure 2.

Figure 9b. Unfolding the above model yields an original configuration in which the irruptive and the Whitewater Groups have the correct facing direction, but the Huronian rocks are completely overturned. Therefore the model in Figure 9a must be rejected.

Figure 9c. A model in which the Huronian rocks have the correct stratigraphic sequence and facing direction, and the granophyre overlies the norite. One objection to this model is immediately apparent: the Stobie Formation is in contact with the granophyre rather than the norite.

Figure 9d. Folding the model in Figure 9c into a syncline places the Huronian rocks within the Sudbury Basin. This model is obviously incorrect.

Figure 9e. Folding the model in Figure 9c into an anticline produces the correct facing direction for the Huronian
rocks, but the funnel shape of the irruptive at the present level of
exposure requires a highly improbable configuration

Figure 9f. A more complex fold model which is consistent with both the facing direction of the Huronian rocks and with the dip directions of the northern and southern portion of the irruptive, but violates the condition that the outcrop pattern of the irruptive must be a simple closed ellipse.

Figure 9g. A model of the Sudbury Basin in which the configuration of the irruptive is a result of faulting. This model is consistent with most of the struc tural features of the Sudbury Basin, but still requires that the granophyre, rather than the norite, be in contact with the Stobie Formation. If the concept of the irruptive as a gravity-differentiated intrusive is valid, this model must be rejected, but this model could be viable if the irruptive should be shown not to be gravity-differentiated. In general, all models in which the irruptive is intruded concordantly with nondeformed Huronian
rocks are inconsistent with the observed structure of the Sudbury Basin, and are
therefore invalid.

(1) Two large dikes, often called "offsets", extend southward from the south range of the irruptive: the Copper Cliff Offset immediately east of the Creighton pluton, and the Worthington Offset 3 kilometers west of the Creighton pluton (Fig. 1). Both dikes extend more than 5 kilometers from the main body of the irruptive, and intrude stratigraphically as far upward into the Huronian rocks as the Mississagi Formation. Both offset dikes are wide, irregular bodies within a kilometer or so of the irruptive, then continue outward as narrow (less than 100 meters wide), extremely straight dikes.

The offset dikes are important in understanding the structural history of the Sudbury Basin because they would be extremely sensitive indicators of relative motion between the south range of the irruptive and the lower Huronian rocks to the south. If the present approximate concordance of the irruptive and the lower Huronian rocks resulted from obliteration of an original angular discordance (Fig. 10), then the rocks of the offset dikes should show considerable internal deformation. In fact, the rocks of both offset dikes are almost entirely nonfoliated, and have clearly not undergone significant internal deformation since their emplacement.

Figure 10 (a-c). Models in which the irruptive is presumed to have cut across flat-lying, nondeformed Huronian rocks, and later brought into conformity with the Huronian rocks through deformation.

Figure 10a. Hypothetical original configuration. Symbols are the same as in Figure 2. Note the dike ("offset") which extends outward from the norite.

Figure 10b. Obliteration of the original angular discordance between the irruptive and the Huronian rocks should result in large shear strains within the offset dike and the Huronian rocks.

Figure 10c. The presence of a large fault along the southern contact of the irruptive should displace the offset dike and produce obvious signs of faulting within the dike rocks. North is on the left in all figures.

There is no evidence to support the conjecture that the irruptive and the Lower Huronian rocks might be separated and displaced relative to one another by a major fault. Both dikes are displaced about a kilometer in a right-lateral sense by the Creighton Fault (Figs. 3, 4), which also displaces the 1600 m.y. old olivine diabase dikes in a similar fashion (Card, 1968). The Copper Cliff Offset is displaced by several other faults (Card, 1969). It is important to note that these fault displacements are small in comparison to the length of the offset dikes, the overall size of the Sudbury Basin, and the thickness of the lower Huronian rocks, and are clearly insufficient to account for the opposite facing directions of the irruptive and the lower Huronian rocks. The observed continuity of the Worthington and Copper Cliff offsets with the main body of the irruptive, and the lack of deformation within the offsets where they join the irruptive preclude the existence of a major fault along the southern contact of the irruptive.

(2) The foliation within the Creighton pluton not only parallels the contact with the Stobie Formation, but also approximately parallels the contact of the nickel irruptive as well. At the northeastern corner of the pluton Se parallels both the contact with the main body of the irruptive to the north and the contact with the Copper Cliff Offset to the east (Fig.). It seems likely that the nickel irruptive was intruded along the original northern contact of the Creighton pluton, and that the intrusion of the Copper Cliff Offset, which follows the eastern contact of the pluton for 2 kilometers, was similarly controlled.

The irruptive probably followed the northern contact of the Murray pluton as well. The Murray pluton is often separated from the irruptive by a thin strip, a few tens of meters wide at most, of metavolcanic rock. (Burrows and Rickaby;: 1934; Cooke, 1946). It seems likely that much of the original northern contact of: the Murray pluton is still extant, and that relics of pre-irruptive country rock are preserved between the Murray pluton and the irruptive. Thus, the field evidence suggests that a substantial part of the south range of the irruptive was structurally controlled by the Creighton and Murray plutons. The concordance of the irruptive with the Stobie Formation and the lower Huronian rocks is probably, at least in part, a consequence of the Creighton and Murray plutons
being concordant with these same rocks.

The above evidence indicates that there has been little or no relative motion between the irruptive and the Stobie Formation, and that the concordance between the irruptive and its wall rocks dates from the time of intrusion of the irruptive. Thomson (1956) recognized that if the petrologic variations within the irruptive were the result of gravitational differentiation, then the southward facing rocks south of the irruptive must have been overturned at the time the irruptive was intruded (Fig. 11). Thomson regarded these geometrical constraints as evidence against gravitational differentiation, and favored a multipleintrusion model. However, it is very difficult to explain the perfectly concentric outcrop patterns of the norite and the granophyre, and the uniformity of the petrographic variations within the irruptive (Naldrett and others, 1970) except in terms of the gravitational differentiation of a single intrusive.

Figure 11 (a-b). Two models of the Sudbury Basin as an impact structure. North-south sections with north on the left; scale and symbols are the same as in Figure 2.

Figure 11a. A model in which the irruptive is an impact melt which differentiates before solidification (Dietz, 1972). The irruptive is approximately concordant with the upturned Huronian rocks beneath the crater. The Onaping Formation covers the still-molten irruptive and mantles the rocks beyond the crater as an ejecta blanket. The remainder of the Whitewater Group has not yet been deposited.

Figure 11b. In this model, the irruptive is viewed as an impact-triggered intrusive. The Whitewater Group rocks partially fill the crater. The Onaping Formation extends beyond the crater as an ejecta blanket and grades downward into brecciated crater wall rocks. The future location of the irruptive is shown by the dashed lines. Non-impact models of the Sudbury Basin are governed by the same geometrical constraints as impact models, and differ only in their interpretation of the lithologic units of the Sudbury Basin. Therefore models
11a and 11b, with some changes in detail, should also describe the pre-irruptive configuration of the Sudbury Basin for non-impact models.

Structural control of the irruptive has obvious economic implications. The nickel-copper ores along the outer contact of the irruptive tend to be concentrated in embayments in the country rock. The major economic properties in the vicinity of the Creighton and Murray plutons are usually located near conspicuous salients in the irruptive, some of which correspond to embayments in the granitic rocks (Card, 1969). Detailed structural studies of the Creighton and Murray plutons may provide clues to the locations of other areas of mineralization; in addition, it is possible that other examples of structural control of the irruptive may exist elsewhere around the Sudbury Basin.

The upturning of the lower Huronian rocks prior to intrusion of the irruptive may have come about in any of the following ways: (1) formation of an upturned rim or central uplift of a meteor-impact crater; (2) post-deposition, pre-irruptive regional deformation of the Huronian rocks; (3) uplift along the flanks of the Creighton pluton diapir, and (4) Archean deformation of the Stobie Formation, with the Huronian rocks above the Stobie Formation being tilted during later deformation events. The field evidence does not permit any of these possibilities to be unequivocally accepted or rejected; nevertheless, some tentative conclusions may be drawn.

(1) A meteor impact large enough to create the Sudbury Basin would certainly result in extensive deformation of the rocks of the crater wall and floor (Dence, 1972). The abrupt steepening (Fig. 2) of the Huronian rocks near the Sudbury Basin strongly re- sembles the upturned rim_ or central uplift of a large impact crater. Whether the Stobie Formation was tilted prior to impact has an important bearing on the interpretation of the Creighton pluton; if the Stobie Formation was flat-lying prior to impact, then the Creighton pluton was originally intruded as a horizontal sill- or laccolith-like body, and later tilted into its present orientation, whereas if the Stobie rocks were tilted prior to impact, the Creighton pluton could have been intruded in essentially its present orientation. If the first alternative is correct, then the present surface of the Creighton pluton represents an originally vertical cross-section of the intrusive (Fig. 13).

Figure 12 (a-b). Pre-impact (a) and post-impact (b) configurations of the Huronian rocks, in which the Stobie Formation is considered to have been originally conformable with the Huronian rocks, and essentially nondeformed prior to impact. The post-impact model shown (b) is the same as that of Figure llb, but the analogous model for Figure lla can easily be envisioned. North is on the left in all figures.

Figure 13(a-c). Possible histories of the Creighton pluton. Symbols are the same as in Figure 2. North is on the left, south on the right in all diagrams.

Figure 13a. The Stobie Formation is flat-lying when intruded by the Creighton pluton (top), and later tilted (bottom). In this case the present section of the pluton by the land surface must represent an originally vertical cross section of the pluton (dashed lines on both diagrams).

Figure 13b. The flat-lying Huronian rocks are carried upward and tilted by the diapiric uprise of remobilized Archean granitic rocks. The present outcrop surface is indicated by a dashed line.

Figure 13c. The Creighton pluton intrudes previously deformed rocks. The deformation may be Archean in age and have affected only the Stobie Formation, or early Proterozoic, and deformed both the Stobie and overlying Huronian rocks. In both models 14b and 14c, the Creighton pluton intrudes in very much its present orientation.

(2) Although penetrative pre-brecciation deformation occurred only in and near the Creighton pluton, the questions of whether pre-Penokean deformation affected the Southern Province, and the tectonic significance of the Creighton and Murray plutons, are still unsettled. (Fig. 14).

Figure 14 (a-c). A similar model to Figure 12, except that the Huronian rocks are deformed prior to the formation of the Sudbury Basin. Model 13c is essentially the same as Figure
11b, except for the preimpact deformation of the Huronian rocks. An analogous history for model lla
can easily be constructed. North on left in all figures.

It is unlikely that the style of intrusion of the Creighton pluton provides any clue as to the tectonic environment in which it was intruded. The heterogeneous, forcefully emplaced Creighton pluton contrasts markedly with the homogeneous, structurally simple Murray pluton, which was apparently not forcefully intruded, yet both plutons are approximately contemporaneous. The granitic rocks of Donegal, Ireland (summarized by Pitcher and Berger, 1972) show a similar range in intrusive styles, though involving many more plutons. The Donegal granites, all related to the Caledonian orogeny, range from drop-shaped, forcefully emplaced diapiric plutons to bodies which were passively emplaced by stoping or by cauldron subsidence, and show no simple relationship between order of intrusion and style of emplacement. Pitcher and Berger noted that the lack of correlation between the order of emplacement and mechanism of intrusion of plutons which were emplaced at about the same crustal level and in the same orogenic event argues against a simple correspondence between intrusion mechanism and tectonic environment or level of emplacement. The contrast in intrusion style of the Creighton and Murray plutons likewise probably reflects differences in mechanical properties of the plutons and their host rocks and may not bear any simple relation to tectonic environment or depth of emplacement. The diapiric intrusion of the Creighton pluton and the high initial Sr87/Sr86 ratios of both the Creighton and Murray plutons (Table I) suggest that both plutons originated through remobilization of sialic crust (Fairbairn and others, 1965; Gibbins, 1972). Although this remobilization need not have been related to any regional tectonic events, there are several several possible contemporaneous events which may be related to the intrusion of the Creighton and Murray plutons. The radiometric ages of the plutons are about the same as that of the Nipissing diabase (Table I). The initial Sr87/Sr86 ratios of the plutons and the Nipissing diabase are so dissimilar that no direct petrologic relationship is likely, but the two sets of intrusions may have been initiated by the same events.

The tholeiitic composition of the Nipissing diabase (Card and Pattison, 1973) suggests that the diabase may be a product of minor back-arc extension related to subduction along the margin of the Superior Province craton. Alternatively, the Nipissing diabase, which is approximately the same age as mafic dikes throughout a large part of the Canadian Shield (Gates and Hurley, 1973) and in Greenland and Scotland (Escher and others, 1976), may be related to a widespread intraplate intrusion event. Either type of event might have resulted in local basement remobili zation. Van Schmus (1976) has reported that pre Marquette Range Supergroup rocks in northern Michigan were metamorphosed about 2100 m.y. ago, and suggested that an orogenic event occurred near the end of Huronian deposition.. The similarity in age of this event with the Creighton and Murray plutons and the Nipissing diabase suggests a possible tectonic origin for the granitic and diabase intrusives.

(3) Experimental models of diapirs have been constructed by Ramberg (1967) and Dixon (1974, 1975). In these experiments, layered media with density inversions are centrifuged, thereby allowing the use of highly viscous model materials which do not deform further once removed from the centrifuge. These experiments have shown that, as a diapir rises, the overlying layers are carried upward along the flanks of the diapir. If the diapir is less viscous than the overlying material, the diapir will pierce this material, whereas if the diapir is more viscous, the material mantling the diapir will be stretched over the top of the diapir, but not pierced (Dixon, 1975). An example of a diapir which is less viscous than its cover material is a salt dome; a mantled gneiss dome (Eskola, 1949) might exemplify a diapir which is more viscous than its mantle (Fig. 13). The steeply plunging structures in the eastern half of the Creighton pluton, and the lack of any evidence of possible feeders of the Creighton pluton at the present level of exposure both support the idea that the pluton was intruded in very much its present orientation. On the other hand, the structure of the Creighton pluton does not rule out the possibility that the pluton was intruded concordantly with flat-lying Huronian rocks and later tilted, as in model (1) above. Because the Stobie and lower Huronian rocks dip steeply along the entire length of the south range, it is unlikely that the intrusion of the Creighton pluton was solely responsible for tilting these rocks.

(4) Cooke (1946) considered the Stobie Formation to be of Archean age, and separated from the Huronian rocks to the south by a "great fault". A normal fault, downdropped on the south, would be consistent with the interpretation of the Huronian depositional basin as a fault-bounded basin and, by juxtaposing older rocks on the north with younger rocks on the south, would be consistent with the present outcrop patterns. (Fig. 15).

Figure 15 (a-b). (Cooke's (1946) model of the Huronian, modified to fit an impact model. North on left.

Figure 15a. Hypothetical pre-impact configuration of the Huronian. Symbols are the same as in Figure 2. The Stobie Formation is part of a greenstone belt, and separated from the Huronian rocks by a "great fault" (GF). Syndepositional motion along the fault has tilted the lower Huronian rocks into approximate concordance with the Stobie Formation.

Figure 15b. Post-impact configuration. The present southern contact of the Stobie Formation is the "great fault". The impact model shown is that of Figure
11b, but an analogous history for Figure 11a
can easily be constructed. More complex histories which combine several of the
simpler models discussed in this paper can also be constructed by analogy with
the appropriate figures.

The structure and lithology of the Creighton pluton and its host rocks are very similar to the structure and setting of plutons in Archean greenstone belts (Clifford, 1972), and the radiometric data for the Creighton and Murray plutons (Table I) can be interpreted to mean that the two plutons are of Archean age with somewhat reset radiometric ages. Thus, the hypothesis that the Stobie Formation is of Archean age need not conflict with presently available geologic data, and provides a simple tectonic explanation for the Creighton and Murray plutons.

Although the hypothesis that the Stobie Formation is part of an Archean greenstone belt has attractive features, it also entails some difficulties. The Stobie Formation must be considered different in age from lithologically similar rocks within the lower Huronian west of Sudbury (Fig. 1). In addition, the radiometric data for Proterozoic rocks in the Sudbury area (Table I), although compiled from numerous sources, show a high degree of internal consistency and are in agreement with the stratigraphic and structural relationships of the lower Huronian rocks. It is doubtful that such consistency would be maintained if the radiometric ages of these rocks had been significantly reset.

The author feels that the best interpretation of the Creighton and Murray plutons is simply that the radiometric ages of these rocks are correct, and that the intrusion of these bodies was related to early Proterozoic tectonic events, of largely unknown nature. Nevertheless, the hypothesis that the Stobie Formation might be Archean is consistent enough with the geology of the Sudbury area to warrant serious examination. Reexamination of the evidence for Cooke's fault and more radiometric dating of the Creighton and Murray plutons and the Stobie Formation are desirable. The Huronian rocks were overturned at the time the irruptive was intruded, probably largely as a result of deformation during the impact event. The extent to which these rocks may have been tilted prior to the impact event cannot be ascertained.